etree.HTML():构造了一个XPath解析对象并对HTML文本进行自动修正。
etree.tostring()：输出修正后的结果，类型是bytes
可参考以下代码：
from lxml import etree
text = '''
<div>
<ul>
<li class="item-0"><a href="link1.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >first item</a></li>
<li class="item-1"><a href="link2.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >second item</a></li>
<li class="item-inactive"><a href="link3.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >third item</a></li>
<li class="item-1"><a href="link4.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >fourth item</a></li>
<li class="item-0"><a href="link5.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >fifth item</a>
</ul>
</div>
'''
html = etree.HTML(text)
result = etree.tostring(html)
print(result.decode('utf-8'))
这里首先导入lxml库的etree模块，然后声明了一段HTML文本，调用HTML类进行初始化，这样就成功构造了一个XPath解析对象。这里需要注意的是，HTML文本中的最后一个li节点是没有闭合的，但是etree.HTML模块可以自动修正HTML文本。
这里我们调用
tostring()
方法即可输出修正后的HTML代码，但是结果是bytes类型。这里利用
decode()
方法将其转成str类型，结果如下
<html><body><div>
<ul>
<li class="item-0"><a href="link1.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >first item</a></li>
<li class="item-1"><a href="link2.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >second item</a></li>
<li class="item-inactive"><a href="link3.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >third item</a></li>
<li class="item-1"><a href="link4.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >fourth item</a></li>
<li class="item-0"><a href="link5.html" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" >fifth item</a>
</li></ul>
</div>
</body></html>
可以看到，经过处理之后，li节点标签被补全，并且还自动添加了body、html节点。
参考：崔庆才的Python3网络爬虫开发实战
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